The present invention relates to an improved optical reticle inspection system and method. The present invention is an improvement of a unit magnification optical system described in U.S. Pat. No. 4,964,705, issued Oct. 23, 1990, and assigned to the same assignee as the present invention, the details of which are hereby incorporated by reference.
Optical reticle inspection techniques are, of course, very important in photolithographic systems used in the formation of semiconductor chips on a wafer.
Prior art approaches in connection with optical reticle inspection techniques have illuminated a small portion of a reticle pattern and re-imaged that small pattern onto a complementary portion of another reticle pattern. Prior art approaches sometimes process one pixel at a time, which is an extremely slow technique.
Such techniques can find defects in the reticle pattern, such as a nick out of a line, a missing corner and the like. However, such prior art approaches depend to a certain extent on the overall pattern integrity and cannot determine whether an edge of the reticle pattern is falling exactly where it should (or not). Unless there is a very evident defect, such as an extra piece of chrome (or a missing piece of chrome), a bite or a spot, prior art approaches have trouble measuring exactly how accurately the pattern of one reticle would overlay that on another reticle.